This invention relates to the catalytic decomposition of ammonia (NH3) from the gas feed to a turbine combustor, as a method to reduce NOx emissions in fossil-fuel fired power plants. In this invention, the efficient removal of ammonia from a gas mixture by catalytic decomposition is achieved by: 1) making a tube-shaped proton-conducting ceramic membrane; 2) coating the inner surface of the membrane with a film of material that will catalyze the reactions: 2NH3xe2x86x92N2+3H2; and, 3) passing the 2H+ ions through the internal space of the coated tube.
NH3 that is present in the gaseous feed to the turbine in a power generating plant represents a small, but significant source, of fuel-bound NOx emitted by such generating plants. In a conventional packed-bed reactor, the rate at which ammonia is decomposed into nitrogen is limited by equilibrium conditions. At temperatures of approximately 600xc2x0 C., only about 53% of the ammonia is decomposed in a conventional packed bed reactor. It should be possible to achieve significant enhancement over the equilibrium conversion rate of the ammonia in the feed stream, if one or more of the reaction products are selectively removed from the reaction zone. In a paper entitled xe2x80x9cCatalytic Decomposition of Ammonia in a Membrane Reactor,xe2x80x9d Journal of Membrane Science 96 (1994) pgs. 259-234, the disclosure of which is incorporated herein by reference, Collins and Way (1994) have shown that ammonia decomposition can be increased to over 94%, at 600xc2x0 C. in a membrane reactor where the chemical conversion and product purification, by separation, take place in the same device. Collins and Way deposited a thin palladium (Pd), membrane on the inside surface of a porous alumina tube and a supported Ni/Al2O3 catalyst was used to decompose ammonia. In the decomposition experiments they conducted, comparing a packed bed Pd-membrane reactor with a conventional packed bed reactor, using a similar catalyst in both situations, ammonia decomposition of over 94% was achieved in the Pd-membrane reactor, as compared with only a 53% decomposition in the conventional packed bed reactor at 600xc2x0 C. In these experiments, hydrogen produced in the reaction was removed from the reaction zone by the thin Pd-film, and this shifted the equilibrium to favor decomposition of NH3. The advantage of the Pd-membrane reactor was even more pronounced at lower temperatures. However, the Pd membrane has several limitations. Most significantly is the fact that the Pd-membrane is poisoned by CO present in excess of 5% of the gas composition. CO is a major constituent of the products of coal gasification. Other limitations include degradation with time at high temperatures, of the hydrogen flux capability of the Pd-membrane and cracks that result from phase transitions occurring at high temperatures. In addition, Pd-membranes are expensive.
Many membrane systems have been developed in efforts to efficiently extract target material from feed streams. Some of these membrane systems (U.S. Pat. Nos. 5,030,661, 5,645,626, and 5,725,633) are synthetic based, and incorporate polymides and polyethersulphones. Unfortunately, such organic membranes are susceptible to chemical damage from H2S and aromatics. Such membranes also have limited temperature tolerance.
Other membrane systems (U.S. Pat. Nos. 4,857,080, 5,366,712, 5,652,020, and 5,674,301) require a multi-component approach wherein a hydrogen permeable metal, such as palladium or platinum overlays a porous ceramic substrate which is provided for strength. Such membranes have limited tolerance to elevated temperatures and are susceptible to chemical reaction to H2S and CO. Furthermore, the multi-component, heterogenous, nature of these membranes adds cost and lessens the reliability of any process which uses them.
Proton-exchange membranes have high proton conductivities, and as such, are currently in development for fuel-cell applications and hydrogen pumps. One such application is disclosed in U.S. Pat. No. 5,094,927, issued to Bauke on Mar. 10, 1992. However, inasmuch as these membranes have relatively low electronic conductivities, they are not viable for hydrogen recovery scenarios, primarily because these membranes require the application of an electric potential to drive proton transport.
We have developed a dense ceramic membrane and have demonstrated the utility of this membrane to separate hydrogen from gas streams containing 33% CO [balance: 66% H2, 1% CO2], at temperatures in excess of 600xc2x0 C. This dense ceramic membrane operates in a non-galvanic mode (without external power circuitry). We have also shown that the hydrogen flux through our ceramic membrane increases as the thickness decreases. We also expect that it will be cheaper to fabricate a ceramic membrane compared to the cost of a Pd-membrane.
It is an object of the present invention to provide a method of decomposing ammonia that overcomes many of the disadvantages of the prior art.
Another object of the present invention is to provide a membrane to extract hydrogen from a myriad of fluids, particularly from the decomposition products of ammonia, aided by the use of a catalyst.
Yet another object of the present invention is to provide a proton- and electron-exchange transfer membrane for use as a reactor for the decomposition of ammonia. A feature of the membrane is that the membrane is a homogenous phase comprised of modified ceramic and a metal to induce electronic conductivity. An advantage of the membrane is that it can be formulated to exhibit tolerance to high temperatures and various chemicals inherent with ammonia-containing feedstream processing, such chemicals including H2O, H2S, CO and CO2. Another advantage is its low cost of fabrication.
Still, another object of the present invention is to provide a hydrogen-transfer membrane having no interconnected porosity with a high selectivity for hydrogen at the exclusion of other non-ionized materials. An advantage of the invention is a two to three-fold increase in hydrogen permeation rates compared to ceramic-based substrates which are not homogenized with electrically conductive materials.
The invention also provides a method for separating hydrogen from the decomposition products of ammonia comprising contacting the decomposition products with a first surface of a membrane to create a solid-gas interface; maintaining the solid-gas interface at a temperature sufficient to allow transport of positive hydrogen ions through the membrane to a second surface of the membrane; and allowing the now-transported positive hydrogen ions to form hydrogen molecules, thereby driving the decomposition reaction of ammonia approaching 94%.